It is frequently desirable, for process control purposes, for regulating the efficiency of a plant or a unit of a plant, for compliance with environmental protection regulations, and for a variety of other reasons, to measure the solids content or concentration of a hot waste gas, e.g. a gas generated by a furnace, or in a chemical or metallurgical plant or elsewhere.
The term "waste gas" is used herein in its broadest sense to refer to any hot exhaust, waste or product gas which may be further processed or simply discharged, no matter how derived. Typical of an exhaust gas with which the present invention is concerned is the gas derived from a thermal power plant boiler, generally prior to gas cleaning and, of course, prior to discharge of the gas into the atmosphere. However, the invention is generally applicable to any system in which determination of the solids content of a hot gas is desirable.
In practice, the measurement of the solids content of a hot waste gas has been effected in the past by diverting a sample of this hot waste gas through a measuring line which includes a measuring filter upon which the solids collect and from which the solids content can be determined, a flowmeter indicating the rate of flow of the sample, and a gas meter signaling the volume of the gas which has been processed.
While this system is effective to provide some indication of the solids content of the waste gas, a problem is encountered when the waste gas temperature is relatively high, since at extremely high temperatures, some of the components which are normally considered to be solids may be in a gaseous or liquid state. Furthermore, simply allowing the sample to cool does not always provide a true indication of solids content because of the different states of aggregation of particles within the gas and the nonuniformity of changes of state which can occur.
The problem is pronounced where the gases contain polycyclic aromatics which tend to accumulate upon the dust particles and undergo changes in composition or state and various transformations at high temperatures.
Thus, earlier systems for determing the solids content of high temperature waste gases were inaccurate when the measurement was effected at high temperature and created additional problems when attempts were made simply to cool the gas sample before measurement.